"I was following a pod of dolphins and whales on my portside and I never thought of looking to the starboard side. About five metres away on my starboard side I saw the fin of the shark. He moved so fast, it was a pretty hard hit, if I didn't have my harness on I would've been thrown overboard."

The colourful character, whose lifelong experiences at sea include being hijacked in 1986 by pirates who stripped his boat, stabbed him in both legs and left him adrift en route to Mombasa, Kenya, had a few theories about why the shark attack happened.

"I'm not sure if it was a bite or just a bump, but it might've been hunting an injured or sick whale, or it could've been attracted to the polycarb rudder extension I'd put on the boat, which thinking about it now, it might've acted like a big lure," the 74-year-old said.

"It was such a hit though. I was so terrified I don't know where he went after he hit me."

The Plymouth, UK-born Rosemount resident is no stranger to shark encounters.

He recalled seeing monster sharks off Thursday Island snatching mackerel off his line, and a day spent bumped about in a two-and-a-half metre life raft after one of his many past vessels, the Shady Lady, sunk in 17 minutes about four years ago.

That capsize had left him and two friends bobbing around the ocean for 18 hours.

"We were hit twice that afternoon by sharks and I reckon what saved us was the aluminium bottom," he said.

"It was my mate's wife's first time at sea but we were picked up by an oil ship before dark.

"Adventure, you've got to have it."

Following the heavy side impact, Mr Charles said he was forced to limp his boat to safety on the inside of Fraser Island to Hervey Bay, where his daughter met him with the trailer to retrieve his now rudderless inflatable.

"It's a brand-new inflatable, I had 100 litres of fuel, food for three-to-four days, I thought I'd done a bloody beautiful job with the boat too," he said.

"I was so cranky; I'd planned the trip for a few months.

"All my grown-up kids said I was mad (heading north in an inflatable), but there's worse things than being called mad."

Mr Charles said he feared the boat would flip after the weight of the impact from what he estimated to have been about a four-metre shark which hit his boat, doing about 15 knots at the time, with such strong force.

"They're amazing creatures," he said.

As for whether he was game to try the journey again, the quirky old sea dog said he would stick to the dry stuff until he gets to Darwin, where he hoped to start up a Thundercat club of his own.